1. Field of the Invention
The present invention relates to a control unit for checking the integrity of tear-off closing devices applied to respective openings in strip packaging material for producing aseptic sealed packages of pourable food products.
2. Background Information
As is known, many pourable food products, such as fruit juice, UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of such a package is the parallelepiped package for liquid or pourable food products known by the name of Tetra Brik Aseptic (registered trademark), which is formed by folding and sealing strip-rolled packaging material. The packaging material has a multilayer structure comprising a layer of fibrous material, e.g. paper, covered on both sides with layers of heat-seal plastic material, e.g. polyethylene. In the case of aseptic packages for long-storage products such as UHT milk, the packaging material comprises a layer of barrier material defined, for example, by an aluminium film, which is superimposed on a layer of heat-seal plastic material and is in turn covered with another layer of heat-seal plastic material defining the inner face of the package eventually contacting the food product.
As is known, such packages are produced on fully automatic packaging machines, on which a continuous tube is formed from the packaging material supplied in strip form. The strip of packaging material is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution; following sterilization, the sterilizing agent is removed, e.g. vaporized by heating, from the surfaces of the packaging material; and the strip of packaging material so sterilized is kept in a closed sterile environment, and is folded and sealed longitudinally to form a tube.
The tube is filled with the sterilized or sterile-processed food product, and is sealed at equally spaced cross sections along which it is then cut into pillow packs, which are subsequently folded mechanically to form the finished, e.g. substantially parallelepiped, packages.
The finished package comprises an opening through which to pour the product; and a tear-off closing device applied to the opening.
The openings are formed and the respective closing devices applied to the strip packaging material before the material is folded into the vertical tube and filled with the food product.
More specifically, each closing device comprises a xe2x80x9cpatchxe2x80x9d element defined by a small sheet of heat-seal plastic material and applied, so as to close the respective opening, to the side of the packaging material eventually defining the inside of the package; and a tear-off tongue applied to the opposite side of the packaging material and heat sealed to the patch element.
The tongue comprises a layer of aluminium; and a layer of heat-seal plastic material, normally polyethylene, which is sealed to the patch element. By virtue of the patch element and the tongue adhering to each other, the portion of the patch element sealed to the tongue is also removed, thus exposing the opening, when the tongue is torn off.
In the course of the heat-seal operation, tears or microholes may be formed in the superheated patch element, particularly as a result of the patch element interacting with the normally fairly irregular edges of the opening formed in the packaging material.
The presence of microholes in the patch element may impair the integrity and asepticity of the package by allowing outside air into the package.
It is an object of the present invention to provide a control unit for checking the integrity of tear-off closing devices applied to respective openings in strip packaging material, and which provides for quality controlling application of the closing devices to the packaging material. It is a further object of the present invention to enable said check to be made during operation of the packaging machine.
According to the present invention, there is provided a control unit for checking the integrity of tear-off closing devices applied to respective openings in a strip packaging material for forming aseptic sealed packages of pourable food products; said packaging material having a multilayer structure comprising at least one electrically conductive sheet of barrier material covered on opposite sides with respective layers of heat-seal plastic material; each said closing device comprising a sheet patch element made of heat-seal plastic material and applied, so as to close a respective said opening, to a face of said packaging material eventually defining the inside of the package, and a tear-off tongue also made of heat-seal plastic material and which is applied to an opposite face of said packaging material and sealed to said patch element; said unit being characterized by comprising:
at least one contact surface at an operating electric potential and cooperating with said packaging material at said patch elements to define one plate of a capacitor, the other plate of which is defined by said sheet of barrier material, and the dielectric of which is defined by the heat-seal plastic material of said patch elements and of the layer of packaging material interposed between said sheet of barrier material and the patch elements; and
detecting means connected to said contact surface and to said sheet of barrier material of said packaging material to detect an electric quantity related to variations in the dielectric strength of the dielectric of said capacitor and so determine the presence of any microholes in said patch elements.